This invention relates to a mine support, to a method of installing the mine support in a mine and removing the support therefrom and also to a pack of components for carrying out the method.
In the mining of coal in the U.S.A. the availablility of increasingly powerful retreat longwall equipment has led to wider and longer longwall panels. Panels greater than 10,000 feet in length and 1,200 feet in width are not uncommon. As gateroads become longer, the rate at which they are mined becomes ever more important. Delays in gateroad mining have caused significant longwall system downtime. In addition the pressure necessary to ventilate the longwall working increases. In an effort to reduce both the gateroad mining time and longwall ventilation pressures, longwall operators mine cut-through entries across the longwall panels in advance of the face. Some longwall mines cut a recovery room entry across the panel at its endpoint to facilitate the removal of longwall equipment. Both cut-through and recovery room entries must be supported with mineable supports to prevent falls of immediate roof in advance of the face. Previously, cut-through and recovery room entries were filled with cement pillars topped with yielding wood cribs or were fully hydraulically backfilled.
The cement pillars have been cast using forms made of cardboard known in the art as Sonotubes. These forms which are cylindrical in shape are therefore bulky and inconvenient to take into the mine from the surface.
A further problem is that the wooden yielding element is difficult to cut using a coal shearer and has on occasions been found to jam the coal face conveyor.
When the entire entries are hydraulically backfilled there is a huge additional burden on the coal preparation plant which separates the waste product from the coal.
The present invention avoids these problems by the provision of a mine support in which the wooden yielding element is replaced with a bag of yieldable grout which bag of grout is cuttable by a coal shearer.
According to the present invention a cuttable mine support is provided comprising (i), a cementitious pillar and (ii) a bag containing a yieldable grout interposed between the top of the pillar and the roof of the mine and wherein said cementitious pillar and bag of yieldable grout are cuttable with a coal shearer.
The cementitious pillar and bag of yieldable grout are both cuttable by a coal shearer and the support of the present invention therefore avoids the problems associated with backfilling or the use of wooden cribs mentioned above.
By the term cuttable we mean that the pillar and the bag of grout may be cut right through by means of a coal shearer, an example of which is a double ended ranging drum shearer made by Joy Technologies Inc.
By cementitous we mean having the property of setting by the addition of water and furthermore being hydraulic by which is meant that it can set under water.
The cementitious pillar may be obtained by the addition of water to a large range of cementitious materials e.g. Portland cement, blast furnace slag and Class C flyash and mixtures thereof.
Other materials such as waste mine tailings, cement kiln dust, crushed rock, Class F flyash or other type of cheap filler that can be made into a hydraulically setting pumpable grout may be used. On the grounds of cost and availablility Class F flyash is usually preferred.
Flyash is defined in Standard Specification C 618 of the American Society for Testing Materials (ASTM) as "finely divided residue that results from the combustion of ground or powdered coal". ASTM C 618 defines two distinct types of flyash, Class F and Class C, the former (obtained by the combustion of anthracite or bituminous coal ) being more common than the latter (obtained by the combustion of sub-bituminous coal or lignite). One characteristic of Class C flyash is its higher calcium content, expressed as lime content and stated by ASTM C618 to be often higher than 10% by weight.
Class C flyash, unless containing a retarder, is inconveniently fast setting and it is therefore preferred not to use Class C flyash alone.
A mixture of Class C and Class F may conveniently be used, the proportions of the mixture being adjusted to give a material which is workable for an adequate period of time. In practice this means that the mixture should be fluid and workable from the time it is mixed with water at the surface until it reaches the form located in the mine where the support is to be installed. This will normally be about an hour.
The flyash is preferably mixed with a cementitious material such as Portland cement to improve its strength, for example as described in U.S. Pat. No. 5,536,310.
Suitable flyash-containing compositions are also described in U.S. Pat. Nos. 5,387,283; 5,439,518; 5,435,843; 5,534,058; 4,992,102; 5,4890,889; 5,556,458; and 5,565,028.
The yieldable grout may be a grout as described in GB Patent No 2,058,037A which discloses a grout which comprises by volume 1 to 30% of a cement mixture, 0.001 to 5% clay and water in excess of 65%, the cement mixture comprising Portland cement, 10 to 75% of a mixture of calcium aluminate and calcium sulphate, 0.5 to 15% of an inorganic salt accelerator and 0.005 to 3% of an organic or inorganic set retarder.
Alternatively the yieldable grout may be one as described in GB Patent No 2,123,808A which discloses a grout which forms Ettringite on hardening and which comprises a high alumina cement, beta-anhydrite and calcium oxide and/or hydroxide. These grouts can contain water to cement ratios up to 2.5:1 or higher by weight. The grout may also be as described in U.S. Pat. No 5,454,866 which discloses a grout containing 20 to 80% of Portland cement, 17 to 35% of a high alumina cement, 20 to 40% of anhydrous calcium sulphate a lithium salt as accelerator for the high alumina cement and a strong base as accelerator for the Portland cement.
Suitable grouts are those known in the art as high yield grouts for example those having a ratio by weight of water to solids of at least 1.5:1 usually in the range 2 to 5:1. The large proportion of water confers the property of being able to release water under pressure and provide the ability to yield in the required manner.
A particularly suitable grout is a high yield grout sold under the Trade Name Tekset by Fosroc International Limited.
A high strength foamed cement would also be suitable as the cuttable yieldable material.
The pillar of cement may be cast using a form which is composed of a plurality of separable parts which permit the form to be removed when the pillar has set and leave a pillar which can be cut.
The form is preferably adjustable in height and this may be provided by the form comprising two (or more) tubular pieces each made of corrugated plastic, steel or iron, the pieces being capable of overlapping longitudinally.
According to another aspect of the present invention there is provided a method of installing a mine support comprising: (i) locating in a mine a form where it is desired to provide support for the mine, the form being composed of a plurality of engaging parts shaped to produce a pillar when filled with a settable material, and the parts being disengageable to permit subsequent removal; (ii) filling the form with a settable cementitious material and allowing the cementitious material to set and thereby produce a pillar; and (iii) removing the form to provide a pillar in the mine which is cuttable with a coal shearer.
The form is conveniently lined with a bag which is to receive the settable cementitious material. The purpose of the bag is to act as a seal and confine the cementitious material within the form and facilitate removal of the form after the cementitous material has set.
The bag may be made of a plastics material such as a coated woven polypropylene. It is possible for the bag to be made of a water permeable material which would allow excess water to escape from the cementitious material through the wall of the bag.
A bag containing a yieldable material may be placed between the top of the pillar and the roof of the mine. A suitable yieldable material is Tekset which is a pumpable cement specially adapted for blocking and wedging cribs and posts and for conforming to roof irregularities sold by Fosroc International. Its setting time is normally about 10 to 15 minutes.
The bag is conveniently of the type which is known in the art used for prestressing a crib. Suitable bags are described in U.S. Pat. Nos. 5,427,476 and 5,288,178.
The flyash cement is conveniently pumped from the surface and the Tekset can be mixed and pumped underground or also pumped from the surface.
According to a further aspect of the invention a pack of components for use in providing a mine support is provided, the pack comprising: (i) a plurality of engageable parts for assembly to provide a form to produce a pillar when the assembled form is filled with a settable material, the assembled parts of the form being disengageable to permit removal of the form from the cementitious material when the latter has set; and (ii) a bag to be placed inside the assembled form to receive the settable material.
The pack may also comprise a bag containing a yieldable grout to be interposed between the top of the form and the roof of the mine.
The form used for making the cementitious pillar of the invention is illustrated by the accompanying drawings.
The form is made up of three tubular sections: a top section, a middle section and a bottom section. Each section is composed of two halves thereby making six main pieces altogether. The halves are held together by right angle shaped steel brackets hereafter referred to as angle irons. Further angle irons join the top and middle sections together and allow different degrees of overlap of these sections thereby providing height adjustment.